1. Field of the Invention
This invention pertains to cutting devices, and more particularly to apparatus for precisely cutting and stripping insulation from insulated electrical conductors.
2. Description of the Prior Art
Industry utilizes two basic types of stripping blades for high production cutting and stripping of insulation from insulated electrical conductors, the "die" type and the "V" type. Although the size, shape, and performance characteristics of wire stripping blades vary greatly within the industry, almost all blades can be classified as either a die or a V type. As a general rule, blades that are designed to form one specific hole size, with no adjustment, are classified as die type. Blade geometries that can tolerate a range of conductor sizes generally fall into the V type category.
V type blades are named for the somewhat V-shaped opening at the working end of the blade. The sloping edges of the V type blade converge on a radius cutting edge that conforms to the periphery of the conductor to be stripped. In practice, a pair of V blades are used such that the V formed in the working ends of the two blades face each other. The blades are reciprocated opened and closed with the working radius cutting edges of the blades bypassing one another. In that way, the opposed respective V-shaped openings of the blades "gather in" and "close about" the insulated conductor.
The significant advantage of the bypassing V blades is that blades of a particular size may be opened or closed to conform to a larger or smaller conductor size. The flexibility of adjustment of the V type blades minimizes costly machine down time, because the blades need not necessarily be changed when a different wire size is to be processed. For that reason, V type blades are the most commonly used blades in the industry. An example of a V type blade is described in U.S. Pat. No. 4,577,405.
There are drawbacks to the V type blade. A significant disadvantage is that there is no guarantee that the blades will cut through the insulation to equal depths as the two V forms bypass each other and penetrate into the insulation. If one blade cutting edge is significantly duller than the opposed blade edge, it is possible that the dull blade will do very little cutting at all. In that case, there is a high probability that the sharper blade will cut too deeply into the insulation and possibly damage the inner conductor.
A second drawback to the traditional V type stripping blade is that the wide gathering V-shaped opening, by definition, prevents the pair of opposed stripping blades from defining the entire 360.degree. of a circular stripping hole. That point was the basis of the invention described in U.S. Pat. No. 4,577,405. The blade described in that patent uses two angles in the V form. A wide angle is used to "gather in" the wire to be cut and stripped. A narrower angle adjacent the radius cutting edge performs the insulation cutting and stripping functions. The narrower blade angle, which is approximately 36.degree. (compared with an angle of approximately 60.degree. used with prior blades), enables a pair of blades to define approximately 80% of the circumference of a circle. Although a significant improvement over the prior blades, about 20% of the blade hole circumference remains non-circular.
Another disadvantage of a pair of bypassing V type blades is that the bypassing cutting edges intrinsically make slices that are not coincident. The "scissor action" of the bypassing blade cutting edges has a natural tendency to bend the insulation before the cutting edges actually penetrate the insulation. If there is a gap between the opposed blades when they close, the result will be two slices into the insulation 180.degree. apart and spaced longitudinally along the wire by a distance equal to the gap between the two blades.
The adjustability feature of the V type blades may be considered to be a drawback. That is because such adjustability requires the machine operator to set the machine to stop the blade end travels. At initial set up, the operator generally does not have a clear indication as to what the hole size will be when the V blades are in the closed position.
It is therefore highly desirable to have a situation in which two blades close to form a very precise and completely circular cutting hole about the conductor, while also providing a means for limiting the depth of penetration of the blade cutting edges into the insulation. The die type blade accomplishes those objectives. The butting nature of the die type blade clearly defines the blade end travel as well as the cutting hole that is formed by the closed butted blades. Examples of prior die type blades include those described in U.S. Pat. Nos. 2,497,112; 2,844,056; 2,871,740; and 4,577,405. U.S. Pat. No. 4,577,405 describes a true die type blade that provides a clearance hole through a pair of opposed blades that is sized to suit the inner conductor as well as to provide clearance for the outer insulation. U.S. Pat. Nos. 2,844,056 and 2,871,740 show another advantage of the die type blade over the V type blade. That advantage is that the two opposed die type blades butt together and interlock with each other in order to assure perfect alignment of the respective blade hole forms.
Unfortunately, there are disadvantages to the die type blade. One disadvantage is that, although a complete circular cutting edge is defined by two butted blades, the blade cutting edge is not sharp around the entire semi-circle of each blade. That is because it is intrinsic to the manufacturing methods employed to produce the die type blade that flats are present at the end points of the semicircular form, as is clearly depicted in U.S. Pat. No. 4,577,405. The flat areas butt together when the opposed die blades close. Therefore, insulation becomes squeezed between the flats of the two closed blades. The squeezed insulation is left uncut and must be torn away during the subsequent stripping stroke. A second disadvantage of the die type blade is that it has poor ability to gather in the insulated electrical conductor to be cut and stripped compared with the V type blade. This disadvantage is explained in detail in U.S. Pat. No. 4,577,405.
In the industry, there are standard conductor gauge sizes (i.e., 20, 18, 16, 14, 12, 10 AWG) that have closely defined diameters. However, the thickness of the insulation applied to the conductors can vary drastically. For example, an 18 ga./19 stranded conductor (having a diameter of approximately 0.052 inches) can have a thin insulation wall of 0.010 inches, a thick insulation wall of 0.100 inches, or any thickness in between. With the prior die blades of U.S. Pat. Nos. 2,844,056 and 2,871,740, a different pair of blades is required for every combination of conductor/insulation sizes.
A further drawback of prior die blades, as exemplified by the blades of the U.S. Pat. Nos. 2,844,056 and 2,871,740 patents, is that they are very difficult to manufacture with close tolerances. Accordingly, the prior design die blades are very expensive.
My co-pending U.S. Patent Application Ser. No. 051,722 describes an invention that provides the combined features of the sharp radius cutting edge, adjustability, and ability to gather in of the V type blade with the interlocking feature of the die type blade. U.S. Patent Application Ser. No. 051,722 also describes an insulation stop as a means for defining the penetration depth of the sharp blade cutting edge. However, that provision can be incorporated into only one of the blades of a pair.
With the foregoing in mind, there exists a need for a wire processing blade that includes the die type features of accurately controlled penetration of the radius cutting edges of both blades into the insulation to prevent damaging the inner conductor and of a completely circular cutting hole, with the V type blade features of good gathering of the wire and of a sharp cutting edge around an entire circle.